st_concavehull function

Applies to: Databricks Runtime 17.1 and above

Returns the concave hull of the input GEOMETRY as a GEOMETRY using the specified length ratio.

Syntax

st_concavehull ( geoExpr, lengthRatioExpr[, allowHolesExpr])

Arguments

• geoExpr: A GEOMETRY value.
• lengthRatio - A DOUBLE value between 0 and 1, inclusive, representing the length ratio used to compute the concave hull.
• allowHoles - An optional BOOLEAN value, indicating whether the output geometry, if a polygon, is allowed to have holes. The default value is false.

Returns

Returns a GEOMETRY that is the concave hull of the input GEOMETRY using the specified length ratio. The resulting geometry, if a polygon, is allowed to have holes iff the third argument is set to true. The SRID value of the output GEOMETRY is equal to that of the input GEOMETRY. An error is returned if the length ratio is not between 0 and 1, inclusive.

The length ratio determines a threshold based on the range between the shortest and longest edges in the Delaunay triangulation of the input points. Edges longer than this threshold are removed from the triangulation. The remaining triangles define the concave hull. For areal input geometries (polygons or multipolygons), the algorithm uses a constrained Delaunay triangulation. The resulting concave hull respects the input GEOMETRY and includes its original polygons.

Examples

> SELECT st_astext(st_concavehull(st_geomfromtext('MULTIPOINT(0 0,10 0,10 10,0 10,1 1,1 5,1 9,5 1,9 9,9 1,9 5,5 9)'), 1))
  POLYGON((0 0,0 10,10 10,10 0,0 0))

> SELECT st_astext(st_concavehull(st_geomfromtext('MULTIPOINT(0 0,10 0,10 10,0 10,1 1,1 5,1 9,5 1,9 9,9 1,9 5,5 9)'), 0.8))
  POLYGON((0 0,1 5,0 10,5 9,10 10,9 5,10 0,5 1,0 0))

Related functions

• st_convexhull function
• st_envelope function